446 A MANUAL OF PHYSIOLOGY 





splits off, or whether a carbonaceous residue may not always 

 hang together for a time and pass through further stages 

 before the carbon is fully oxidized. We shall see that under 

 certain conditions some of the carbon of proteids may be 

 retained in the body as glycogen or fat ; and this suggests 

 that in all cases it may run through intermediate products as 

 yet unknown, before being finally excreted as carbon dioxide. 



2. Metabolism of Carbo-hydrates Glycogen. The carbo- 

 hydrates of the food, passing into the blood of the portal vein 

 in the form of dextrose, are in part arrested in the liver, and 

 stored up as glycogen in the hepatic cells, to be gradually given 

 out again as sugar in the intervals of digestion. The proof of 

 this statement is as follows : 



Sugar is arrested in the liver, for during digestion, espe- 

 cially of a meal rich in carbo-hydrates, the blood of the 

 portal contains more sugar than that of the hepatic vein. 

 In the liver there exists a store of sugar-producing material 

 from which sugar is gradually given off to the blood, for 

 in the intervals of digestion the blood of the hepatic vein 

 contains more sugar (2 parts per 1,000) than the mixed 

 blood of the body or than that of the portal vein (i to 1*5 

 part per 1,000). When the circulation through the liver is 

 cut off in the goose, the blood rapidly becomes free, or 

 nearly free, from sugar (Minkowski). And a similar result 

 follows such interference with the hepatic circulation as is 

 caused by the ligation of the three chief arteries of the 

 intestine in the dog, even when the animal has been pre- 

 viously made diabetic by excision of the pancreas (p. 483). 

 The nature of the sugar-forming substance is made clear by 

 the following experiments : (i) A rabbit after a large carbo- 

 hydrate meal, of carrots for instance, is killed, and its liver 

 rapidly excised, cut into small pieces, and thrown into acidu- 

 lated boiling water. After being boiled for a few minutes, 

 the pieces of liver are rubbed up in a mortar and again 

 boiled in the same water. The opalescent aqueous extract is 

 filtered off from the coagulated proteids. No sugar, or only 

 traces of it, are found in this extract; but another carbo- 

 hydrate, glycogen, an isomer of starch giving a port-wine 

 colour with iodine and capable of ready conversion into 



